The present invention relates to an apparatus for driving a fluorescent display tube of the type in which the thermoelectrons emitted from the filaments forming the cathodes impinge against the anodes to cause the fluorescent substance on the anodes to emit light, thereby making the display tube suitable for use in general industrial applications.
This type of known fluorescent display tube has been disadvantageous in that as shown in FIG. 7, when the thermoelectrons emitted from filaments 2 impinge on anodes 4, the thermoelectrons emitted from the different filaments lie one upon another on the surface of the anodes 4 so that variations are caused in the degree of emission and relatively light and dark portions are produced. Methods have heretofore been proposed to overcome this problem of emission irregularity and one such method is disclosed in Japanese Patent Publication No. 59-24486. This method is schematically shown in FIG. 8 in which filaments 2 emit thermoelectrons to anodes 4 and an antistatic layer 40 is provided adjacent to each anode 4. The same positive potential as the anodes 4 with respect to the filaments 2 are always applied to a transparent electrode or diffusing electrode 1 and the antistatic layers 40 so that an electric field is produced and the densities of the electron streams flowing from the filaments 2 to the anodes 4 are diffused and averaged, thereby accomplishing a reduced brightness irregularity. However, this construction is not capable of preventing the occurrence of brightness irregularity in a display tube using no antistatic layers 40 and therefore a more effective brightness irregularity preventive measure is required. Also, there is another prior art method disclosed in JP-A No. 57-205943. As shown in FIG. 9, this method employs a plurality of filaments 2 which are each arranged to flow current in the opposite direction to that of the adjacent one and to thereby make uniform the potentials at the right and left ends of the filaments. In other words, while a phenomenon is caused in which there is a difference in potential between the right and left ends of each filament due to the voltage effect of the current flowing therein, the direction of current flow is changed alternately so that this potential unbalance is eliminated and a stream of electrons flows uniformly from each filament to the corresponding anode.
With the prior art method of JP-A No. 57-205943 shown in FIG. 9, however, the directions of currents i.sub.1 and i.sub.2 flowing in the filaments are made opposite to each other and this requires a special wiring, thereby complicating the construction. Also, there exists a need for a construction capable of accomplishing a greater reduction in the emission irregularity than that attained by the construction of FIG. 9. Further, there is a need for accomplishing a reduced emission irregularity by another construction different from the previously mentioned prior art constructions of FIGS. 8 and 9. Still further, there is a need for a fluorescent display tube which employs the construction of FIG. 9 and an additional construction to reduce the variations in emission to a greater extent.